In a general wireless communication system, although a bandwidth in an uplink and a bandwidth in a downlink are set different from each other, a single carrier is mainly taken into consideration. For instance, based on a single carrier, it is able to provide a wireless communication system having uplink and downlink bandwidths symmetric to each other in general with a single carrier for configuring an uplink and a single carrier configuring a downlink.
In order to support a bandwidth extended more than that of an existing wireless communication system, it is able to apply a carrier aggregation technology for bring an effect of using a logically wind band by combining a multitude of bands together in a frequency region. In a carrier aggregation supportive system, a cell can be understood as a combination of a downlink and an uplink. In defining a cell of a currently defined wireless communication system, an uplink resource is not an essential element. Hence, a cell can be configured with a downlink resource only or both a downlink resource and an uplink resource. The downlink resource can be called a downlink (DL) component carrier (CC). And, the uplink resource can be called an uplink (UL) component carrier (CC). The DL CC and UL CC can be represented as a carrier frequency. And, the carrier frequency means a center frequency of a corresponding cell.
Cells can be categorized into a primary cell (PCell) operating at a primary frequency and a secondary cell (SCell) operating at a secondary frequency. PCell and SCell may be commonly called a serving cell. In brief, the PCell is the cell that becomes a control related center, while the SCell is the rest of the serving cell except the PCell.
A user equipment is able to perform a random access procedure to obtain an uplink transmission timing alignment value or achieve various objects. In an existing wireless communication system, when a base station sends a random access response in response to a random access preamble transmission of a user equipment, information on at least one user equipment is provided through a single random access response. Hence, one user equipment monitors a control channel addressed with a prescribed identifier in a search space configured in common with another user equipment, decodes a data unit transmitted on a data channel indicated by the control channel, and is then able to find a random access response for the corresponding user equipment based on a preamble identifier included in the data unit.